The process traditionally used for the commercial production of the economically most important alkoxysilanes involves two main steps. The first of these entails the synthesis of silicon tetrachloride. This can be made in a variety of ways. In one, silica is reduced to elemental silicon with carbon in an electric arc furnace, and this is then chlorinated with elemental chlorine. The reduction of the silica to silicon consumes large amounts of electrical energy, a fact reflected in the cost of the silicon. Once the silicon tetrachloride has been prepared, it is reacted with an alcohol, such as ethanol, to produce the tetraalkoxysilane: EQU SiCl.sub.4 +4C.sub.2 H.sub.5 OH.fwdarw.Si(OC.sub.2 H.sub.5).sub.4 +4HCl
An economically important alkoxysiloxane is made by the hydrolysis of tetraethoxysilane. It contains about 40 wt% SiO.sub.2 and is often called ethyl silicate 40.
Various alkoxysilanes can be made by a transesterification process: ##STR1## Among the catalysts used for this process are sodium alkoxides. This process is used mainly for the preparation of the alkoxysilanes of alcohols with relatively high boiling points.
The so-called direct process for producing alkoxysiloxanes involves two steps. In the first, silica is reduced to elemental silicon with carbon in an electric arc furnace. Then it is treated with an alcohol, such as ethanol, to produce the tetraalkoxysilane. Both the traditional and direct processes are energy intensive. They require complete scission of the silicon-oxygen linkages and formation of new ones.
Calhoun and Masson (J. C. S. Dalton 1980, 1282) found that hexaisopropoxycyclotrisiloxane is formed as a very minor byproduct when pseudowollastonite, Ca.sub.3 Si.sub.3 O.sub.9, is treated with trimethylchlorosilane, hexamethyldisiloxane, and isopropyl alcohol. This work shows that alkoxysiloxanes can be made from silicates as byproducts under the reaction conditions used by the authors.
It has also been shown by Bleiman and Mercier (Inorg. Chem. 1975, 14, 2853) that the sheet silicate chrysotile, Mg.sub.3 Si.sub.2 O.sub.5 (OH).sub.4, (common asbestos) can be partially converted to a partially esterified sheet silicate by treating it with hydrochloric acid and isopropyl alcohol and then treating the resulting material with allyl alcohol and pyridine.
It has been reported by the present inventors, Polym. Prepr. Am. Chem. Soc. Div. Polym. Chem. 1986, 27, 107, that wollastonite, CaSiO.sub.3, can be heated at a temperature below its melting point for an extended period of time to produce the cyclotrisilicate, pseudowollastonite (Ca.sub.3 Si.sub.3 O.sub.9), which upon treatment with acid and ethanol produces octaethoxytrisiloxane and hexaethoxycyclotrisiloxane. It was also taught by the present inventors in the Polym. Prepr. Am. Chem. Soc. publication that high yields of specific siloxane species can be obtained depending on the crystal structure of the silicate starting material. For example, it has been taught by the present inventors that Ca.sub.3 Si.sub.3 O.sub.9 has the potential to give high yields of the cyclic trimeric siloxane upon treatment with acids and alcohols.
Earlier work by the present inventors utilized a metal salt of silicon dioxide, a cyclic silicate, or a linear oligomeric silicate, but did not proceed through the specific and preferred cyclic metal halide salt intermediate described herein.
Chukhlantsev, in Dokl. Phys. Chem. (Engl. Transl.) 1979, 246, p.530, reported work related to the synthesis of the metal silicate halide salt, Ca.sub.8 Si.sub.4 O.sub.12 Cl.sub.8, from quartz but not from wollastonite. Chukhlantsev was directed primarily to the formation of Ca.sub.2 SiO.sub.3 Cl.sub.2.
United Kingdom patent number 732,533, issued to Wacker-Chemie G.m.b.H. on June 29, 1955, teaches and claims the reaction of a polysiloxane containing silicon-bonded hydrocarbon radicals and alkoxy or aryloxy groups with sodium, lithium, calcium, magnesium or zinc, or a mixture thereof, and a halogenated hydrocarbon.
U.S. Pat. No. 4,309,557, issued on Jan. 5, 1982 to Compton et al., teaches and claims the process for displacing alkoxy groups on linear oligomeric siloxanes by a Grignard reaction to produce short chain, linear alkylated siloxanes.
Kuroda et al. reported the synthesis of polyorganosiloxane retaining the silicate framework from inosilicate mineral, para-wollastonite, by trimethylsilylation. Kuroda et al., Polymer, 1978, vol. 19, November, p. 1300-1302.
The present inventors filed on Oct. 4, 1985, a related patent application titled "Silicate Esters And Organosilicon Compounds", Ser. No. 784,216.
A need exists for an economic process not based on complete reduction and subsequent re-oxidation of silicon, which will produce in high yield cyclic tetrameric alkoxysiloxanes and cyclic tetrameric alkylsiloxanes.